As say it's name, emit particles from a geometry. For further informations about particles systems, go check this part of the doc : Particles
This modifier can be added over an existing particle system to make some trails. But this can also be done with Emit
. Go check the Particle Rule
part of the doc.
Beware that number of emitted particles represent the total of emitted particles. If you put this modifiers with a rate of 30 particles/seconds over o system containing millions of particles, you'll get 30 particles/second as a result.
Check this quick tutorial to see a little bit an extensive use of this modifier. This tutorial was made on an earlier version of Smode so the "place" modifier you see in use is the actual 3D Transform
Particles simulation speed can be adjust through their Cue Transport